1. Field of the Invention
The present invention concerns a large carbon refractory, which is suitable as lining material for the side walls and bottom sections of blast furnace basins, and a method of manufacturing such a large carbon refractory.
2. Description of the Prior Art
Carbon refractories are generally manufactured by adding organic binder, such as coal-tar pitch, phenol resin, etc. to a carbon aggregate, such as coke, artificial graphite, flake graphite, amorphous graphite, or calcined anthracite, and then kneading the mixture and forming the mixture by extrusion or compression molding and thereafter baking the molded product in coke breeze packing.
Carbon refractories have excellent thermal conductivity and resistance to slag in comparison to fireclay bricks. Since it is also easy to manufacture large blocks from carbon refractories, they have been used from the past in the lining of the basin part of blast furnaces. However, the longevity of carbon refractories have not reached satisfactory levels.
The causes of damage of carbon refractory linings within blast furnace include carburization dissolution into the molten iron, structural destruction accompanying the penetration of molten iron into the pores and temperature fluctuation, formation of cracks due to the penetration of and reaction with alkali and zinc vapors and formation of cracks due to thermal stress, etc.
Various proposals concerning the formulation, manufacturing conditions, usage methods, etc. of carbon refractories have been proposed and implemented in order to extend the longevity of carbon refractories. The present applicant has also disclosed, in Japanese patent publication No. Sho-56-18559, blast furnace carbon refractories, with a low molten iron erosion rate, obtained by selecting calcined anthracite of low molten iron erosion rate (1/7 that of pitch coke, 1/4 that of artificial graphite, 1/2 that of amorphous graphite) and adding 2-30% of metal oxides such as .alpha.-alumina, zircon, and magnesia to this anthracite.
The present applicant has also disclosed, in Japanese patent publication No. Sho-58-43350, a method of manufacturing blast furnace carbon refractories, in which 5-15 parts of metallic silicon particulates and 15-25 parts of coal-tar pitch are added to 75-85 parts of carbon aggregate mainly composed of anthracite and the mixture is kneaded, molded, and baked so that whisker-like silicon compounds form within the pores of the carbon refractory to lessen the pores with a pore size of 1 .mu.m or more into which molten metal can penetrate and to thus reduce the penetration of molten iron and reactive gases.
Furthermore, the present applicant has disclosed, in Japanese patent publication No. Sho-61-3299, a method of manufacturing blast furnace carbon refractories characterized by kneading a mixture composed of 40-60 parts of flake graphite with a particle size of 0.3-3 mm, 15-30 parts of artificial graphite with a particle size of 0.1-4.5 mm, 10-20 parts of silicon carbide with a particle size of 0.074 mm or less, and 5-15 parts of metal powder upon adding organic solvents and phenol resin solution as a binder, then by adding phenol resin powder and then kneading further, and then by molding, drying, and baking.
The carbon refractory made by the above manufacturing method was satisfactory, having a large thermal conductivity and small permeability and suffering little molten metal penetration and carburization dissolution. However, since the flake graphite was used as it is in the flake form, its strong orientation properties presented a big disadvantage. That is, although the properties in the direction parallel to the alignment of particles were excellent, the thermal conductivity and bending strength in the perpendicular direction were inadequate. Furthermore, although medium-sized blocks could be manufactured with this carbon refractory, when it came to manufacturing large blocks such as blocks with a size of 600.times.700.times.2500 mm, the spring back during molding was large and laminations and cracks formed easily, making the product yield low.
In order to prevent the damaging of blast furnace carbon refractories, it is required that; (1) the carburization dissolution into the molten iron be made small, (2) the pores be made small to reduce the penetration of molten metal and reactive gases, and (3) the thermal conductivity be made high to reduce destruction by thermal stress.
The carbon refractory inside a blast furnace is not damaged uniformly and the level of damage differs according to the location. It is preferable to use carbon refractories of high thermal conductivity at sections of severe damage and to increase the safety level of such sections by locating the 1150.degree. C. solidification line of the molten iron away from the shell. The addition of artificial graphite or flake graphite is effective in increasing the thermal conductivity of anthracite refractories with a thermal conductivity of 13 W/(m.K). However, artificial graphite aggregates have many large pores with a pore size of 1 .mu.m or more into which molten iron may penetrate easily and the level of carburization dissolution into the molten iron is also high. With flake graphite aggregates, laminations tend to occur easily due to the strong orientation and the manufacturing of large blocks is difficult as described above.
In general, a particle size formulation, composed of 20-45% of coarse grains with particle size of 1-5 mm or 1-10 mm, is used for aggregates for manufacturing large refractories for blast furnaces, etc. If a particle size formulation contains a low amount of coarse grains, the large heat shrinkage in the baking process will lower the product yield by causing baking cracks and will thus make stable manufacturing of large blocks difficult. If flake graphite is used as it is in the flake form as a coarse-grain aggregate, it will present problems such as spring back during molding. Furthermore, if coarse grains of artificial graphite are added as they are, the coarse artificial graphite grains at the machined surface of the blast furnace refractory will dissolve preferentially into the molten iron, causing the surface to be eroded in a pitted manner and leading to the progress of erosion into the interior of the structure.